This invention relates to a golf ball which can be manufactured with ease of working and exhibits good resilience characteristics.
To confer golf balls with outstanding resilience characteristics, various improvements were made in the prior art in formulating the polybutadiene used as the base rubber.
For example, U.S. Pat. No. 4,683,257 proposes a rubber composition for use in solid golf balls, comprising a polybutadiene having a Mooney viscosity of 70 to 100 and synthesized using a nickel or cobalt catalyst, in admixture with another polybutadiene having a Mooney viscosity of 30 to 90 and synthesized using a lanthanoid catalyst or polybutadiene having a Mooney viscosity of 20 to 50 and synthesized using a nickel or cobalt catalyst as the base rubber.
However, the composition of the above patent needs further improvements in workability during extrusion and resilience.
U.S. Pat. No. 4,955,613 proposes golf balls prepared using a blend of a polybutadiene having a Mooney viscosity of less than 50 and synthesized with a Group VIII catalyst in combination with a polybutadiene having a Mooney viscosity of less than 50 and synthesized with a lanthanide catalyst. However, the resulting golf balls have poor resilience characteristics.
U.S. Pat. No. 6,312,346 proposes a multi-piece solid golf ball having an intermediate layer formed of a low-Mooney viscosity polybutadiene; JP-A 11-319148 proposes a solid golf ball molded from a rubber composition comprising a polybutadiene having a Mooney viscosity of 50 to 69 and synthesized using a nickel or cobalt catalyst in combination with a polybutadiene having a Mooney viscosity of 20 to 90 and synthesized using a lanthanoid catalyst; U.S. Pat. No. 6,194,505 proposes a solid golf ball molded from a rubber composition based on a rubber having a 1,2 vinyl content of at most 2.0% and a weight-average molecular weight to number-average molecular weight ratio Mw/Mn of not more than 3.5; U.S. Pat. No. 4,929,678 proposes a golf ball molded from a rubber composition comprising a high Mooney viscosity polybutadiene; and U.S. Pat. No. 5,082,285 proposes a golf ball molded from a rubber composition comprising polybutadiene having a high number-average molecular weight in admixture with polybutadiene having a low number-average molecular weight. However, all these proposals have drawbacks including difficult working during extrusion and inferior resilience characteristics of the resulting golf balls.
U.S. Pat. No. 4,546,980 describes use of two organic peroxides, and U.S. Pat. No. 4,770,422 describes use of amino amount of organic peroxide. However, there arise problems including insufficient resilience and a prolonged crosslinking time causing a substantial lowering of productivity.
In general, polybutadienes synthesized using rare-earth catalysts are believed to be highly resilient, but leave a problem of extremely inefficient working.
In the prior art, studies for achieving effective working are being made by varying the blend ratio of different polybutadienes while there remains a problem that a polybutadiene having ease of working is added at the sacrifice of resilience.
For conventional polybutadienes synthesized using Group VIII catalysts, there remains a problem that if the amount of organic peroxide added thereto is reduced, a longer vulcanization time is needed, resulting in a lowering of productivity.